1. Field of the Invention
The present invention relates to a sieving device with a supporting frame which can be made to vibrate with a vibration drive associated with and exclusively acting upon the supporting frame, and a vibration frame held against the supporting frame via spring elements and spring-elastically coupled to the supporting frame, in which the vibration frame can vibrate in at least essentially free vibration in one direction of vibration (of tension waves), and in which the supporting frame and the vibration frame comprise transverse supports which are alternately arranged one behind the other in the direction of vibration of tension waves and which can carry sieve bottom elements which can alternately be tensioned and untensioned in the direction of vibration of tension waves. Further, the present invention relates to a method for operating such a sieving device. The invention particularly relates to such a sieving device used for materials which are difficult to sieve, such as moist, sticky, caking materials and/or materials containing long fibres.
2. Description of Related Art
German Patent DE 35 03 125 C2, which forms the starting point of the present invention, discloses a sieving device with the characteristics mentioned above, comprising a supporting frame which can be made to vibrate, and a vibration frame held to the supporting frame by spring elements so as to be able to vibrate. The transverse supports of the vibration frame extend outwardly through window-like apertures in lateral side plates of the supporting frame, where they are connected to longitudinal supports of the vibration frame. Spring elements in the form of thrust-type rubber blocks are arranged preferably at several locations on each longitudinal support in the longitudinal direction, opposing each other in pairs on both sides of a longitudinal support. The thrust-type rubber blocks represent the only bearing arrangement of the vibration frame on the supporting frame, so that the vibration frame can carry out a vibration movement in relation to the supporting frame; said vibration movement being determined by the thrust-type rubber elements and being largely linear. To generate vibration, a vibration drive, which in particular is in the form of a circular vibration drive, is associated with the supporting frame, said circular vibration drive acting exclusively on the supporting frame and only indirectly via the thrust-type rubber blocks on the vibration frame. Thus, the vibration frame can vibrate freely. Such a known sieving device has a number of disadvantages.
The spring constant of thrust-type rubber blocks greatly depends on the temperature of the thrust-type rubber blocks. Accordingly, the vibration behavior greatly depends on the temperature.
Thrust-type rubber blocks are relatively prone to ageing. Correspondingly, the vibration behavior changes over time; in particular, it worsens.
During vibration, relatively substantial internal friction in the thrust-type rubber blocks must be overcome, resulting in a substantial heating effect. However, the thrust-type rubber blocks must not exceed certain temperature limits or otherwise they harden. Accordingly, in the known sieving device, the extent of vibration, i.e., amplitude and frequency, or average speed and acceleration, is very limited. The vibration frequency achievable over an extended period is essentially a maximum of 800 vibrations per minute.
By fixing the longitudinal supports between the thrust-type rubber blocks which are arranged so as to oppose each other in pairs, the vibration forcesxe2x80x94i.e., vibrationsxe2x80x94transverse to the essentially horizontal vibration of the tension waves of the vibration wave, are to be suppressed so that essentially only linear vibration takes place. However, in the known sieving device, superposition of transverse vibration cannot be prevented altogether, so that the thrust-type rubber blocks additionally vibrate in this transverse direction, and thus, additionally heat up. This has the effect of additionally reducing the vibration frequency and/or vibration amplitude which are available at the most for vibration of tension waves, i.e., movement of the sieve (bottom) elements.
Usually during sieving, the material to be sieved is to be conveyed in one sieving direction along the sieve surface formed by the sieve bottom elements. To this effect, the vibration acting on the supporting frame normally has a respective main direction of effect (direction of sieving). In order to achieve adequate conveyance in the direction of sieving, an inclination of the vibration frame or of the direction of vibration of tension waves is required. In the known sieving device, the usually straight shape of the longitudinal supports of the vibration frame require an angle of inclination of the vibration frame of approx. 12xc2x0 to 16xc2x0 in relation to horizontal, so as to achieve adequate conveyance in sieving direction. Due to the design length of such a sieving device, this requires an undesirably high design height.
German Patent 1 206 372 discloses a similar sieving device with a vibration frame which can be made to vibrate relative to a supporting frame, thus causing alternate tensioning and untensioning of sieve bottom elements which are held by transverse supports of the frames. Preferably, a vibration device is provided which acts directly on both the supporting frame and the vibration frame so that a compulsorily or positively controlled, circular vibration of the vibration frame relative to the supporting frame is generated. For this purpose, springs acting between the supporting frame and the vibration frame are exclusively used to provide movable support or guidance.
In contrast to the already mentioned free vibration of the vibration frame, compulsorily or positively controlled vibration generation involves increased design expenditure. In addition, the above-mentioned sieving device is associated with disadvantages similar to the sieving device according to German Patent DE 35 03 125 C2.
German Patent 1 206 372 describes two alternative embodiments in which the vibration frame is held so as to vibrate freely, i.e., the vibration drive does not act directly on the vibration frame. In this embodiment, the vibration frame is held by spring elements which are rubber blocks installed between longitudinal supports of the vibration frame and the supporting frame. In addition, as is the case in German Patent DE 35 03 125 C2, these spring elements are exposed to shear forces. Correspondingly, these alternative embodiments also involve disadvantages as in German Patent DE 35 03 125 C2.
It is a main object of the present invention to provide a sieving device with a freely vibrating vibration frame of the xe2x80x9cvibration of tension wavesxe2x80x9d type, as well as a method for operating such a sieving device, which allows more effective sieving, in particular of materials which are difficult to sieve, wherein the device and method in particular avoid or at least minimize the disadvantages of the prior art while allowing a simple, compact design.
The above object is achieved by a sieving device in which the supporting frame and the vibration frame comprise transverse supports which are alternately arranged one behind the other in the direction of vibration and which are adapted to carry sieve bottom elements which can be tensioned and untensioned in the direction of vibration, the spring elements being helical springs with main spring axes that are at least essentially aligned in the direction of vibration, and wherein, in addition to the spring elements, guiding elements are provided for bearing or holding the vibration frame at the supporting frame.
A significant idea of the present invention lies in providing helical spring elements acting in the direction of vibration (of tension waves), for movable coupling between the vibration frame and the supporting frame, and furthermore, in providing additional guiding elements which hold or guide the vibration frame at least essentially freely in the direction of vibration (of tension waves). This results in several advantages.
The helical spring elements, which are preferably made of steel, can withstand very considerable loads, and accordingly, they withstand vibrations at high amplitude and high frequency even over extended periods. Accordingly, higher acceleration during tensioning of the sieve bottom elements, and consequently, an increased or improved sieving performance, can be achieved. Greater acceleration is also beneficial for conveying the material to be sieved in the sieving direction. This is an additional reason for being able to reduce the inclination of the vibration frame and/or the direction of vibration in the sieving device according to the invention, when compared to the prior art, for example, to 3xc2x0 to 5xc2x0 relative to horizontal. This results in a correspondingly considerably lower design height.
The helical spring elements provided are significantly less sensitive than thrust-type rubber blocks. In particular, the helical spring elements are quasi temperature-independent, thus providing for more universal application, without there being any danger of overheating. In addition, the service life of helical spring elements is significantly longer than those of thrust-type rubber blocks. Consequently, there is a reduction in costs of operation and maintenance.
A further significant aspect of the solution according to the invention is that the guide elements provided in addition to the spring elements, hold or guide the vibration frame such that it is essentially free, and is at least movably guided essentially in the direction of vibration of the sieve. Thus, the desired vibration movement can easily be specified in an optimal way, with the spring coupling between the supporting frame and the vibration frame being able to be set and realized independently thereof. Accordingly, at least in essence, a linear vibration movement of the vibration frame, relative to the supporting frame, can be specified resulting in a main direction of vibration. This is beneficial for an optimal sieving effect and good conveyance in the sieving direction. It also essentially prevents the spring elements from being exposed to shear forces; a factor which is beneficial to their service life.
When the main direction of effect of the vibration drive extends diagonally to the direction of vibration, as is usual, in particular, to achieve good conveyance of the material to be sieved in the sieving direction, the guiding elements can prevent interfering vibrations of the vibration frame transversely to the direction of vibration.
A further aspect of the present invention provides for the sieving device to be operated or to be operable at an increased vibration frequency compared to the prior art, namely, at least 850 vibrations per min., and in particular, at approx. 890 vibrations per min. In this way, in particular, an average speed of the vibration frame relative to the supporting frame of preferably at least 11 m/min and an average acceleration of preferably at least 2.2 m/s2 is achieved. In this way, a very strong vibration can be achieved which allows more effective sieving. Furthermore, better conveyance in the sieving direction of the material to be sieved is achieved, so that the inclination of the vibration frame or the direction of vibration can be significantly reduced, in particular to 3xc2x0 to 5xc2x0 relative to horizontal. Correspondingly, the required design height of the sieving device can be reduced.
Further details, characteristics, properties, advantages and objects of the present invention are shown in more detail below by means of the drawing of a preferred embodiment.